This invention generally relates to systems for measuring areas of residual stress in structures and, more particularly to ultrasonic systems for making such measurements.
In the manufacture of large structures such as pressure vessels a number of large welds are required. After welding the heat affected zones surrounding these welds usually contain residual stress due to uneven cooling rates. One procedure that is widely performed to relieve these residual stresses is so heat the entire structure to an appropriate temperature and thereafter to carefully control its rate of cooling.
At the present time there is no satisfactory test for actually measuring the success of this stress relief procedure. Now and then high residual stress regions exist in the structure after completion of the procedure and if these regions occur in critical areas, cracks develop and fracture can occur.
Currently, the standard non-destructive testing procedures consist of radiography and ultrasonic pulse echo. Neither of these techniques, however, can reveal the presence of residual stress. Radiography records only the variations in the specific gravity of the material being tested. Ultrasonic imaging cannot be used to reveal areas of residual stress because these regions are not sharply defined and hence do not reflect sufficient acoustic waves to be measured.
It is well known that the velocity of sound in a solid is affected by the residual stresses. This phenomenon is a third order effect and has been used as a research tool to determine the Lame and Murnaghan elastic constants for various materials. It is also known that the elastic constants are related to the velocity of sound in solid materials.